Support variable expansion for paths in Vault configuration

Problem to solve

The Vault Integration does not support paths set from variables, and it is a viable option for setting paths of secrets

Further Details

From @tmaczukin in slack:

Usage of variables in Vault configuration indeed is not fully supported (or at all supported) and this is something that we should definitely look on. @jreporter If you remember my issue and first PoC about passing secrets between jobs in the pipeline, $CI_PIPELINE_ID variable was used to prepare the path of the secret that was being used. In my PoC this worked because operations against secrets were executed totally different and expansion of the variables was made automatically (don't want to go into details now 😉). When we've decided to change the way how we handle secrets reading, this was lost and I've totally forgot to test this workflow. So IMHO this

Proposal

Support variable expansion for the Vault: keyword

secrets:
  foo:
    vault: "foo/$CI_COMMIT_BRANCH/bar/baz@kv"